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Colemanite

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A large loose cluster of colemanite crystals without matrix. Some cleaving on the backside, in good condition from the front.The discovery of colemanite within Death Valley, located in Inyo County, California, United States, represents a watershed moment in the history of the American borax industry. Geologically, this hydrous calcium borate mineral is hosted within the Miocene-to-Pliocene-age Furnace Creek Formation, a thick sequence of lacustrine sedimentary rocks, mudstones, and tuffaceous sediments deposited in an ancient, restricted basin characterized by intense volcanic activity. Hot, boron-rich hydrothermal fluids discharged into these playa lakes, initially precipitating primary borates like ulexite, which later underwent extensive diagenetic alteration and groundwater leaching to recrystallize into dense, tabular beds and spectacular geodes of water-clear, lustrous colemanite crystals. Historically, the mineral was first discovered in Death Valley in October 1882 by prospector Philander Banister, who found massive outcroppings in the Furnace Creek Wash, leading to immediate claim staking and the naming of the species after William T. Coleman, owner of the Harmony Borax Works. Commercial underground mining began in earnest around 1883 and expanded dramatically under the Pacific Coast Borax Company, which developed major operations like the Biddy McCarty and Lila C mines to extract the high-grade ore, hauling it out across the desert using the legendary twenty-mule teams until the construction of the Death Valley Railroad in 1914 modernized transport. Large-scale corporate extraction of colemanite in the immediate Furnace Creek district ran continuously for over four decades until 1927, when the discovery of even larger, lower-cost sodium borate deposits near Boron, California, shifted company focus and caused the permanent cessation of regular mining in the valley.